As for a speaker reproduction system that reproduces an audio signal such as music or announcement with a speaker, there are some cases where the quality of sound deteriorates because of distortion or crackling noise. The cause of the distortion or crackling noise is divided broadly into two categories. The first is the case where the input signal to a speaker is distorted, and the second is the case where the distortion or crackling noise occurs because the input signal exceeds the reproduction limit of a speaker even though it does not have distortion.
The first case can be explained as follows. Recently, an increasing number of audio signal reproduction systems correct the frequency characteristics or adjust the volume by digital processing. As for the correction of the frequency characteristics, if a high frequency component is increased by 10 dB, for example, there is a possibility that the digital signal saturates at the volume not less than −10 dBFS. Here, 0 dBFS represents the maximum amplitude of the digital signal. Accordingly, the reproduced sound is digitally distorted at high volume so that the quality of sound deteriorates. FIG. 2 shows its behavior.
In FIG. 2, the vertical axis shows the amplitude intensity of the digital signal, and the horizontal axis shows the frequency. In addition, the region where the signal is saturated and the crackling noise occurs is shown by gray, and the border is denoted by a thick line. Reference numerals 201, 202 and 203 designate an example of frequency characteristics of the digital audio signal with its frequency characteristics being corrected: 201 denotes the characteristics when the volume is low; 202 designates the characteristics when the volume is moderate; and 203 denotes the characteristics when the volume is high. At the volume designated by 201 or 202, since the audio signal does not exceed 0 dBFS, no crackling noise occurs, which enables enjoying the original quality of sound. However, if the volume is increased as denoted by 203, the signal intensity of a part of the high frequency component exceeds 0 dBFS and is saturated digitally. If the signal saturates, it causes distortion or crackling noise, which deteriorates the quality of sound.
Thus, when trying to reproduce the digital signal with its frequency characteristics being corrected at a high volume, there are some cases where a particular frequency component exceeds 0 dBFS, the maximum amplitude of the digital signal. This will cause the saturation of the signal, which brings about the distortion or crackling noise.
Next, the second case will be described which will bring about distortion or crackling noise because of exceeding the reproduction limit of the speaker.
Concerning the speaker reproduction, the diaphragm of a speaker has a maximum displacement it can vibrate. If a signal exceeding it is input, the speaker diaphragm cannot vibrate well, which can cause distortion or crackling noise. Here, the displacement of the speaker diaphragm depends on the frequency of the input signal. FIG. 3 shows the relationships. FIG. 3 shows the displacement of the speaker diaphragm when a signal is input to a speaker while varying only its frequency with its voltage (V) being maintained. In FIG. 3, although the actual characteristics around F0 vary in such a manner as to become rounder or flatter than those of FIG. 3 because of the difference in the Q-value representing the damping degree, a general tendency is consistent. In addition, since the present invention is applicable to a speaker with characteristics different from the displacement characteristics shown in FIG. 3, the following description will be made on the assumption that the displacement characteristics of the speaker diaphragm are as shown in FIG. 3 for the sake of convenience of description.
As shown in FIG. 3, the displacement of the speaker diaphragm is approximately constant at frequency components lower than F0 (the minimum resonant frequency of the speaker), and decreases with a slope of about −12 dB/oct for the frequency components higher than F0. This indicates that the speaker diaphragm vibrates with a greater displacement when the lower frequency components near F0 are input to the speaker than when the higher frequency components are input. Accordingly, when the signal including a lot of low frequency components are input to the speaker and its voltage is raised, its displacement will exceed the maximum displacement of the diaphragm at a certain voltage or higher. In other words, as the signal includes more low frequency components and its voltage increases, it is likely that the displacement will exceed the reproduction limit of the speaker. FIG. 4 shows the behavior.
In FIG. 4, the vertical axis shows the amplitude intensity of the signal, and the horizontal axis shows its frequency. Besides, the region where the displacement exceeds the displacement limit of the speaker diaphragm and hence the crackling noise can occur is shown in gray, and its border is indicated by a thick line. Here, since the characteristics of FIG. 4 are characteristics for the amplitude of the audio signal, the displacement limit of the speaker diaphragm has a slope of +12 dB/oct which differs from the slope of the characteristics of the displacement of the speaker shown in FIG. 3.
In addition, reference numerals 401, 402 and 403 denote frequency characteristics of the audio signal reproduced with the speaker on the assumption that the signal includes a lot of low frequency components in particular. The reference numeral 401 designates the characteristics when the volume is low, 402 denotes the characteristics when the volume is moderate, and 403 designates the frequency characteristics when the volume is high. As long as the reproduction is carried out at the low volume as indicated by 401, even the audio signal including a lot of low frequency components does not cause the speaker diaphragm to exceed its maximum displacement. Thus, the crackling noise does not occur, which enables enjoying the original quality of sound. However, as denoted by 402 or 403, when the volume is raised, the speaker diaphragm will exceed its maximum displacement, which can cause distortion or crackling noise to occur, and deteriorate the quality of sound.
In this way, when the signal that will cause the diaphragm to exceed the maximum displacement is input, the diaphragm cannot vibrate well, thereby resulting in the distortion or crackling noise.
Since the distortion or crackling noise is not contained in the original audio signal, they will greatly hamper enjoying music.
With regard to this problem, the distortion or crackling noise is conventionally reduced by a processing configuration as shown in FIG. 17. In FIG. 17, the configuration passes an input signal 1301 through a HPF (high-pass filter) 1302 that suppresses the low frequency components, and outputs a signal 1303. Since the configuration can suppress the low frequency components that will cause the crackling noise before inputting to the speaker, it can reduce the ratio of producing the distortion or crackling noise. However, since the conventional technique suppresses the low frequency components through the HPF 1302, even when the signal to be reproduced has only a small amount of low frequency components and the crackling noise does not occur when driving the speaker with high voltage, since the low frequency components are always suppressed, it offers a problem of being unable to reproduce the original sound. In addition, even when the crackling noise will not occur without passing through the high-pass filter 1302 because the driving voltage of the speaker is not so high, the conventional technique always suppresses the low frequency components, which offers a problem of being unable to reproduce the original sound. In other words, the conventional technique has a problem of hampering a listener from enjoying the original quality of sound by excessively suppressing the low frequency components to prevent the crackling noise at the large voltage driving (at the high volume).
As a technique to relieve the problems, there is a technique disclosed in Patent Document 1, for example. FIG. 18 is a block diagram of the processing of an amplitude limiting device disclosed in a Patent Document 1. According to the Patent Document 1, in the amplitude limit for suppressing the excessive input, it detects the amount of distortion due to amplitude limit characteristics, and controls the gain of each frequency band in accordance with the amount, thereby reducing the deterioration of the quality of sound due to the amplitude limit.